darwin.linux-builder: Disable evaluation

A remote builder does not need to evaluate anything, so let's trim
it down to (eventually) save some space, and make the purpose of
the builder clear.

Users should evaluate on the host instead.

nixos/modules/profiles/macos-builder.nix

@@ -103,6 +103,16 @@ in
     # server that QEMU provides (normally 10.0.2.3)
     networking.nameservers = [ "8.8.8.8" ];
 
+    # The linux builder is a lightweight VM for remote building; not evaluation.
+    nix.channel.enable = false;
+    # remote builder uses `nix-daemon` (ssh-ng:) or `nix-store --serve` (ssh:)
+    # --force: do not complain when missing
+    # TODO: install a store-only nix
+    #       https://github.com/NixOS/rfcs/blob/master/rfcs/0134-nix-store-layer.md#detailed-design
+    environment.extraSetup = ''
+      rm --force $out/bin/{nix-instantiate,nix-build,nix-shell,nix-prefetch*,nix}
+    '';
+
     nix.settings = {
       auto-optimise-store = true;